The present invention relates to a method and device that correct artifacts.
In industrial computed tomography (CT), scatter radiation also occurs in addition to the primary radiation that is to be detected. If nothing is done to prevent the detection of scatter radiation or if the recorded projections remain uncorrected, this leads to scatter radiation artifacts in the reconstructed CT volume composed of voxels. Scatter radiation artifacts of said type can be caused for example as a result of what is called a cupping effect which leads to inhomogeneous voxel values in a homogeneous object material, such that when the density values are plotted along a line, in other words when a line profile is produced, a curve results instead of a straight line. Generally, streak patterns and contrast losses can be produced in addition.
Different conventional solution approaches to scatter radiation correction exist which can mainly be classified into two groups:
1. Measures to reduce the detected scatter radiation, as implemented for example through the use of an anti-scatter grid.
2. So-called a posteriori corrections of the scatter radiation, whereby the scatter component in each CT projection is subtracted accordingly.
A maximally precise knowledge of the detected scatter component is necessary for the second group. Toward that end various approaches for determining said scatter component exist, which can likewise be divided into two groups:
1. Software-based solutions, such as Monte Carlo simulations, deterministic calculations of first-order scatter, convolution algorithms based on what are known as point spread functions, for example.
2. Experimental methods for determining the scatter component with the aid of measurements.
Within this second group of experimental methods, different measurement methods are known. For example:
a) beam-stopper-based methods, and
b) a complementary technique thereto, which uses apertures and moreover so-called beam holes, and
c) an only recently proposed method which is based on what is termed primary modulation.
With regard to the last-cited method, the following related art is known.
U.S. Pat. No. 7,463,712 B2 discloses a scatter correction method for x-ray imaging, wherein a direction-dependent modulation of the primary x-ray radiation is used, leading to a space-dependent modulation on the primary radiation detector. Scatter radiation in an x-ray imaging system including an x-ray source and an x-ray detector is corrected by using amplitude modulation to translate the spatial frequency of a detected x-ray beam to a higher frequency and by filtering out the low-frequency scatter radiation. A measure for the low-frequency primary signal without scatter radiation is then obtained by demodulating the detected modulated signal.
The decisive advantage of a method using primary modulation over other conventional experimental methods is that the scatter measurement and scatter beam estimation can be carried out during the actual CT scan, i.e. the scatter data is acquired simultaneously with the actual CT projections. Compared with other conventional methods that require an additional measurement operation, this results in a smaller measurement overhead and at the same time realizes a time saving, which represents a critical advantage in particular for industrial CT. Furthermore, savings in terms of radiation dose are also achieved compared to measurement methods in which an additional measurement operation is required.
The conventional method according to U.S. Pat. No. 7,463,712 B2 provides that a primary modulator is placed between the object that is to be imaged and the x-ray tube. The primary modulator impresses a pattern, for example in the form of a checkerboard composed of light and dark fields, on the primary beams by amplitude modulation. For that purpose a printed circuit board made of copper for example can be used, into which a pattern is introduced by etching processes, i.e. the copper is correspondingly etched away on the light fields. The different strengths of the attenuation properties or attenuation coefficients of copper and of the bare printed circuit board material ensure a corresponding beam attenuation through the dark fields (copper), while there is hardly any attenuation or none at all on the light fields (printed circuit board material). Throughout the entire CT scan or the entire CT sampling the modulator remains stationary between object and x-ray tube, i.e. it does not change its position. The modulated checkerboard-like pattern is therefore to be found again in every projection of the CT scan, i.e. both in free beam regions and in object-covered regions. In this case the relative modulation strength, insofar as the primary signal is concerned, is equally great at all points. However, not just this modulated primary signal is recorded by the detector, but in addition, superimposed on said signal, is a low-frequency scatter signal which results due to x-ray scatter effects, in particular Compton scatter processes, in the test object and in the laboratory environment. The detector therefore records an overall signal formed of the modulated primary signal and the superimposed unmodulated scatter signal.
Subsequently it is possible to separate the modulated primary signal from the unmodulated scatter signal in the Fourier domain. This is accomplished by corresponding high-pass or low-pass filtering of the modulated projection. The low-pass filtered version of the modulated projection results in the overlapping of the frequency components of unmodulated primary image and scatter function in the frequency domain. The high-pass filtered version contains only the spectral component of the modulated primary signal, in other words it can subsequently be demodulated and weighted in order to obtain an approximation of the exclusive primary signal in the frequency domain. Following an inverse Fourier transform this can be subtracted as an approximated primary image from the low-pass filtered version, which includes scatter and primary signals, in order to obtain an approximation of the scatter image. It is pointed out that in the method described herewith so-called edge detection and edge smoothing, also referred to as boundary detection, is applied to every modulated projection. Smoothing of said kind takes into account that high-frequency image components are already present due to the object alone and in particular due to the object edges. This is independent of any modulation. Said unmodulated high-frequency components overlap in the Fourier domain with the spectral copies of the modulated primary signal. Mixing modulated and unmodulated signal will, if the latter is not corrected, result in incorrect demodulation of the primary signals. In other words, artifacts will be produced especially in the object edge regions and also in the inside of the object, which is then referred to in this context as “spilling”, leading to incorrect reconstruction of the primary image. In order to attenuate or suppress such high-frequency image components, which are caused in particular by object edges, the above-cited boundary detection is applied in order to find the object edges and smooth the same accordingly by a Gaussian filter.
Although the strong artifacts caused by excessively high-frequency, unmodulated image components are reduced in this process, an inaccuracy in the edge region is produced at such points, since the Gaussian filtering is no longer reversed subsequently. The thus obtained scatter image is now subtracted accordingly from the CT projections. Since the modulation pattern is still now present in the projections, the CT projections are normalized to the radiation intensity after the modulator. In this way the modulator pattern is removed in the projection image. This is accomplished by division by a recording of the modulator without further objects in the beam path. It is pointed out that beam hardening effects are produced due to the modulator, and moreover these are caused in particular by the dark copper fields. If such effects remain uncorrected, the result is firstly an inaccurate scatter estimation and secondly an incomplete removal of the checkerboard pattern in the last-cited division step. This can lead to ring artifacts in the CT cross-sectional images. The fact that beam hardening effects exist and lead to the cited errors is known for example from “Correction for beam hardening artifacts in computerized tomography,” by Hammersberg et al. (Journal of X-Ray Science and Technoglogy 8, 1998).
Hammersberg et al. discloses that when polyenergetic x-ray sources are used, accurate density measurements are made more difficult due to beam hardening based on incorrect gradients of linear attenuation coefficients in computed tomography cross-sectional images. A correction method is described in which polyenergetic computed tomography data is converted into monoenergetic computed tomography data by linearization. Computed tomography data is derived from measured data points as a function of the object thickness and represented as a polynomial. Using simulations, the polyenergetic computed tomography data is accurately simulated on the basis of the object material density, the object material composition, the x-ray energy spectrum, the detector response, and the information transfer from the detector to digitized data. The curve of the function representing the polyenergetic computed tomography data can be accurately determined by an eighth- or higher-order polynomial or by cubic spline interpolation.